Method of production of synthetic fluorite having a selected grain size

ABSTRACT

Method of production of synthetic fluorite with a selected specified grain size to be obtained, by submitting the calcium carbonate to a reaction with fluoride ions present in an aqueous solution, consists therein that calcium carbonate is used having a grain size substantially the same as the selected grain size for the synthetic fluorite to be obtained. The fluoride ions for the reaction with the calcium carbonate are supplied in the form of a solution of ammonium fluoride and/or potassium fluoride and/or sodium fluoride. The process is conducted at a temperature chosen within the range of 50° C to 150° C, under a pressure reduced, atmospheric or elevated, within the range of 0.5 to 10 atm, and preferably at the boiling temperature or at a temperature not too far from the boiling point, especially in a reaction system containing ammonium fluoride. The process is conducted by mixing the calcium carbonate grains with a solution containing fluoride ions, or by passing the solution containing the fluoride ions through a bed of grains containing the calcium carbonate. In both cases it is preferable to conduct the process by counter-current displacement of the reaction agents.

This invention relates to a method of production of synthetic fluorite-- calcium fluoride -- having a selected predetermined grain size withinthe grain size range from 1 micrometer to several score millimeters,Said fluorite is applicable for many purposes in the chemical industryand in metallurgy.

There is known a method of production of synthetic fluorite by thereaction of ammonium fluoride with calcium carbonate according to theformula:

        2 NH.sub.4 F + CaCO.sub.3 = CaF.sub.2 +  (NH.sub.4).sub.2 CO.sub.3    

in this process a solution of ammonium fluoride obtained from thehydrolysis of fluosilicic acid is used.

There is also known the synthetic preparation of calcium fluoride by thereaction of calcium carbonate with sodium fluoride and potassiumfluoride:

        2 NaF + CaCO.sub.3 = CaF.sub.2 + Na.sub.2 CO.sub.3                            2 kf + caCO.sub.3 = CaF.sub.2 + K.sub.2 CO.sub.3                      

in all said reactions calcium carbonate was used, with very fine grainsize, due to its higher reactivity. It was assumed thereby that thereaction occurs exclusively on the surface of the calcium carbonategrains with separation of the crystalline phase of the product from thecrystalline phase of the parent substance. In all known processessynthetic fluorite is produced, showing a significant dispersion, andbeing characterized by bad filtering ability. For this reason theproduction thereof is difficult from a technical point of view, and theapplication of a fine-size grained product is limited.

In the course of investigations of the conversion of calcium carbonatewith ammonium fluoride, sodium fluoride or potassium fluoride using thereaction of a solution containing one, two or three of said salts, itwas unexpectedly found that the produced calcium fluoride -- syntheticfluorite -- may be produced with a selected grain size, and even in theform of pieces. The grain size of the fluorite depends on the grain sizeof the calcium carbonate used in the reaction, whereby in the producedfluorite the size and shape of the grains of the calcium carbonate arereproduced. Even such an unexpected process of conversion of the calciumcarbonate into the fluorite can be explained -- in the light of theinvestigations carried out to explain that phenomenon -- by thereduction of the volume of the crystalline phase in the course of theconversion, and by forming therefore free spaces in the grains wheremovements of the carbonate and fluoride ions occur.

The method, according to the invention, of production of syntheticfluorite with a a selected grain size, by submitting the calciumcarbonate to a reaction with fluoride ions present in an aqueoussolution, consists therein that calcium carbonate is used having aspecific grain size, such as predetermined for the fluorite to besynthetically prepared. The fluoride ions for the reaction with calciumcarbonate are used in the form of a solution of ammonium fluoride and/orpotassium fluoride and/or sodium fluoride.

The process is conducted at a temperature chosen within the range of 50°C to 150° C, under a reduced, atmospheric, or elevated pressure withinthe range of 0.5 to 10 atm. A mixing is applied of the calcium carbonategrains with a solution containing fluoride ions, preferably not toointensive mixing, or the solution containing the fluoride ions is passedthrough a bed of calcium carbonate grains. In both cases it ispreferable to conduct the reaction in counter-current stream of thereactants. It is also preferable to conduct the process at the boilingpoint or at a temperature lying not significantly far from the boilingpoint, especially in a reaction system containing ammonium fluoride.

In some cases it is advantageous to introduce an industrial gascontaining silicon tetrafluoride and/or hydrogen fluoride directly intothe system containing the calcium carbonate, and a solution containingammonia and/or ammonium carbonate and/or potassium carbonate and/orsodium carbonate and to separate the obtained post-reaction suspensionwith isolation of the synthetic fluorite, the silica and of the solutionof the ammonium carbonate and/or sodium carbonate and/or potassiumcarbonate, which are than returned to the process.

The method according to the invention renders it possible to obtainsynthetic fluorite with a very small content of the calcium carbonate,with almost complete passing of the fluorite from the solution into thecrystalline phase. The substantial advantage of the method according tothe invention is also the fact that in the reaction a solution offluoride can be used, containing silica suspended therein, as the silicaobtained from the silicon tetrafluoride can be easy separated from thecoarse-grained fluorite, insuring the product purity.

EXAMPLES

The mineral calcium carbonate used in the reaction had a small amount ofimpurities with respect to the quality of fluorite obtained therefrom.

In the reaction mineral calcium carbonate was used, containing 98 -98.5% CaCO₃ and 0.3 - 0.7% SiO₂, and being moreover contaminated withmagnesium carbonate and aluminum and iron compounds.

Calcium carbonate was used, having different, determined grain sizeswithin the range of 1 μm to 20 mm.

For the reaction with calcium carbonate solutions of ammonium fluorideand potassium fluoride were used. A high grade solution of ammoniumfluoride and potassium fluoride was used, obtained by ammonia hydrolysisof potassium fluosilicate, which in the reaction with the calciumcarbonate gives, besides the fluorite, a solution of potassiumcarbonate.

EXAMPLE 1

Calcium carbonate with a grain sized of 1 - 20 μm, used in the amount of100 g, was heated for 5 hours at a temperature of 50° - 70° C with asolution of ammonium fluoride, containing 222 g NH₄ F in 1 liter (6mol/l), used in the amount of 340 ml.

A reaction mixture was obtained containing in 1 liter about 5 g ofammonium fluoride and fluorite with grain size of 1 - 20 μm, containingabout 3% of calcium carbonate.

EXAMPLE 2

Calcium carbonate with a grain size of 0.2 - 0.4 mm, used in the amountof 100 g, was heated for 2 hours at a temperature of 95° - 98° C with asolution of ammonium fluoride, containing 222 g of NH₄ F in 1 liter,used in amount of 340 ml.

A reaction mixture was obtained, containing in the 1 liter about 3 g ofammonium fluoride and fluorite mainly with grain size of 0.2 - 0.4 mm,containing about 1 % of calcium carbonate.

EXAMPLE 3

Calcium carbonate with grain size of 0.2 - 0.4 mm, used in the amount of100 g, was heated for 1 hour at a temperature of 95° - 98° C with asolution of potassium fluoride, containing 348 g of KF in 1 liter (6mol/l), used in the amount of 340 ml.

A reaction mixture was obtained, containing in 1 liter about 122 g ofpotassium fluoride. On further heating for 4 hours at the sametemperature a solution was obtained, containing in 1 liter about 40 g ofpotassium fluoride.

The obtained fluorite had a main grain size of 0.2 to 0.4 mm. Thecontent of CaCO₃ in the fluorite was about 13%.

EXAMPLE 4

Calcium carbonate with a grain size of 0.2 - 0.4 mm, used the amount of100 g, was heated for 21/2 hours at a temperature of 95° - 98° C with asolution of potassium fluoride and ammonium fluoride in the molar ratioKF/NH₄ F = 1/2, containing in 1 liter 264 g of both salts, used in theamount of 340 ml.

A reaction mixture was obtained, containing in 1 liter about 11 g ofpotassium fluoride, and fluorite with main grain size of 0.2 - 0.4 mm,containing about 6% of calcium carbonate.

EXAMPLE 5

Calcium carbonate with a grain size of 0.2 - 0.4 mm, used in an amountof 100 g, was heated for 4 hours at a temperature of 95° - 98° C with asolution of potassium fluoride, containing 116 g of KF in 1 liter, usedin the amount of 1 liter.

A reaction mixture was obtained, containing in 1 liter about 21 g ofpotassium fluoride, and fluorite with a main grain size of 0.2 - 0.4 mm,containing about 22% of calcium carbonate.

EXAMPLE 6

Calcium carbonate with a grain size of 0.2 - 0.4 mm, used in the amountof 100 g, was heated for 5 hours at a temperature of 95° - 98° C with asolution of the potassium fluoride and ammonium fluoride in the molar1/2 ratio KF/NH₄ F, containing in 1 liter 88 g of both fluorides, usedin amount of 1 liter.

A reaction mixture was obtained, containing in 1 liter about 9 g ofpotassium fluoride and ammonium fluoride, and fluorite with main grainsize of 0.2 - 0.4 mm, containing about 12% of CaCO₃.

EXAMPLE 7

Calcium cabonate with a grain size of 8 - 20 mm, used in the amount of100 g, was heated for about 120 hours at a temperature of 95° - 98° Cwith a solution of ammonium fluoride, containing in 1 liter 222 g of NH₄F, used in the amount of 1 liter.

Fluorite was obtained with a main grain size of 7 - 18 mm, containingabout 7% of CaCO₃.

EXAMPLE 8

The fluorite grains, having the size mainly of 0.2 to 0.4 mm, andcontaining about 22% CaCO₃, obtained in the process as specified inExample 5, and used in an amount of 80 g, was heated for 2 hours at atemperature of 95° - 98° C with a solution of ammonium fluoride,containing in 1 liter 111 g of NH₄ F, used in amount of about 200 ml.

Fluorite was obtained with main grain size of 0.2 - 0.4 mm, containingabout 2% of CaCO₃.

EXAMPLE 9

Calcium carbonate with a grain size of 0.7 - 1.1 mm was used in theamount of 100 g, and heated for 10 hours at a temperature of 95° - 98° Cwith a solution of potassium fluoride and ammonium fluoride in a molarratio KF/NH₄ F = 1/2, containing in 1 liter 264 g of both salts, used inthe amount of 340 ml.

A reaction mixture was obtained, containing in 1 liter about 8 g ofpotassium fluoride and ammonium fluoride, and fluorite with a main grainsize of 0.5 - 1.0 mm, containing about 4% of CaCO₃.

EXAMPLE 10

Calcium carbonate with a grain size of 0.7 - 1.1 mm, used in the amountof 100 g, was heated for 5 hours, at a temperature of 120° - 130° C,under a pressure of about 2 atm, while removing the reaction gases, witha solution of potassium fluoride and ammonium fluoride in a molar ratioKF/NH₄ F = 1/2, containing in 1 liter 264 g of both salts, used in theamount of 340 ml.

A reaction mixture was obtained, containing in 1 liter 7 g of potassiumfluoride and ammonium fluoride, and fluorite with a main grain size of0.6 - 1.0 mm, containing about 2 % of CaCO₃.

EXAMPLE 11

Fluorite grains obtained as specified in Examples 1, 3, 4, 5, 6 and 9were heated for 0.5 - 2 hours, depending on the grain size, at atemperature of about 95° C with solutions of ammonium fluoride orpotassium fluoride and ammonium fluoride, with a concentration of thefluoride ions of 6 R in 1 liter.

Fluorite was obtained, containing 1 - 2% of CaCO₃.

What is claimed is:
 1. A method for the production of synthetic fluoritehaving a controlled selected grain size in the range from about 1 μm to20 mm by reacting 1 mole of calcium carbonate having substantially saidselected grain size with an aqueous solution of 2 to 6 moles of afluoride selected from the group consisting of ammonium, potassium andsodium fluorides at a temperature in the range of 50° C. to 150° C. fora period of 1 to 120 hours to produce fluorite grains of said selectedgrain size which reproduce the size and shape of the grains of thecalcium carbonate.
 2. The method of claim 1 wherein the reaction iscarried out at a pressure in the range of 0.5 - 10 atmospheres.
 3. Themethod of claim 1 wherein the reaction is carried out at substantiallythe boiling point of the solution.
 4. The method of claim 3 wherein thefluoride is ammonium fluoride.
 5. The method of claim 2 wherein thecalcium carbonate is mixed with the fluoride solution.
 6. The method ofclaim 2 wherein the fluoride solution is passed through a bed of calciumcarbonate.
 7. The method of claim 5 wherein the calcium carbonate andthe fluoride are reacted countercurrently.
 8. The method of claim 5wherein the fluoride solution is formed in situ by passing a gasselected from the group consisting of silicon tetrafluoride, hydrogenfluoride and mixtures thereof into a system comprising the calciumcarbonate and a solution of a compound selected from the groupconsisting of ammonia, ammonium carbonate, potassium carbonate andsodium carbonate.
 9. The method of claim 8 wherein the gas comprisessilicon tetrafluoride and the reaction product comprises a suspension ofsilica and the fluorite in a solution of ammonium, potassium or sodiumcarbonate and wherein the silica is separated from the fluorite grainsand the carbonate solution from the reaction product is recycled. 10.The method of claim 6 wherein the fluoride solution is formed in situ bypassing a gas selected from the group consisting of silicontetrafluoride, hydrogen fluoride and mixtures thereof into a systemcomprising the calcium carbonate and a solution of a compound selectedfrom the group consisting of ammonia, ammonium carbonate, potassiumcarbonate and sodium carbonate.
 11. The method of claim 10 wherein thegas comprises silicon tetrafluoride and the reaction product comprises asuspension of silica and the fluorite in a solution of ammonium,potassium or sodium carbonate and wherein the silica is separated fromthe fluorite grains and the carbonate from the reaction product isrecycled.
 12. The method of claim 1 wherein the calcium carbonate has agrain size of 0.2-0.4 mm.
 13. The method of claim 1 wherein the calciumcarbonate has a grain size of 8-20 mm.
 14. The method of claim 1 whereinthe calcium carbonate has a grain size of 0.7 to 1.1 mm.